The production of silicon-based electronic devices requires silicon in which the density of defects in its crystal lattice structure is within low limits. Several different types of defects can be harmful. The presence of Group III or Group V electronically active impurities, such as aluminum, arsenic, boron, phosphorus, and the like can create so-called extrinsic point defects. Other types of defects, for example intrinsic point defects, line defects, stacking faults, and grain boundaries, can occur even in the absence of electronically active impurities, when the silicon is processed, even at mildly elevated temperatures. These latter types can be referred to as non-electronic impurity defects.
In contrast to the well understood extrinsic point defects, the causes of non-electronic impurity defects are not so clearly defined. Events leading to potential or actual crystal defects that would impair or ruin the properties of an electronic device made from silicon can occur at various stages, in the formation of the elemental silicon itself or later. For example, these defects can occur in the thermal decomposition or reduction of silicon-containing compounds. They can also occur in the production of single crystal silicon from molten silicon, for example in the Czochralski or float zoning techniques, and they can arise in the production of devices from previously formed elemental silicon, as in diffusion processes, expitaxial growth of monocrystalline silicon layers, ribbon forming processes, and in the silicon on ceramic process.
One group of crystal defects of the non-electronic impurity type are known as oxygen and carbon-related defects. Neither carbon nor oxygen is an electronically active impurity. Nevertheless, if the density of oxygen- and carbon-related crystal defects is too high in a given device, those defects can be a cause of improper or inadequate electronic performance.
Both oxygen and carbon are easily transported through the gas phase in the silicon processing environment, for example in the form of CO.sub.2, CO, SiO or H.sub.2 O.sub.(v). Because oxygen can also combine chemically with silicon to form solid SiO.sub.2, it has generally been the practice to attempt to exclude it from the chamber in which silicon is processed. Thus the common procedure has been either to evacuate the chamber or to blanket the chamber under an inert gas, such as argon or helium. However, such procedures nevertheless leave significant amounts of oxygen in the processing environment. The residual oxygen content may be many orders of magnitude greater than the extremely low values required to prevent oxygen from reacting with silicon. For example, at 1700.degree. K., at which silicon is a liquid, the oxygen partial pressure in an evacuated or inert gas filled silicon processing chamber may be only 10.sup.-6 atm but oxygen related defects can still occur. In fact, silicon can be undesirably oxidized at any oxygen partial pressure above the equibrium pressure for oxygen-silicon reactions by which silicon oxides are formed; at 1700.degree. K. this pressure is about 10.sup.-18 atm. The presence of carbon dissolved in, or in the environment of, molten silicon, reduces the oxygen partial pressure by reaction with the oxygen and therefore increases the amount of oxygen which can be tolerated in the ambient gas without detrimental oxygen-related defects being formed upon solidification of the silicon. At the same time, the carbon content of the molten silicon may be lowered, reducing the potential for carbon-related defects in the solid crystal.
Large proportions of carbon are often present in the chamber. Heaters, crucibles, and other structures in the processing environment are often made of graphite because of its useful high temperature properties. The reactions between carbon and oxygen provide products which transport carbon as well as oxygen through the vapor phase to the silicon surface and from there into the body by diffusion.